This application is related to copending application Ser. No. (08/270,331) filed concurrently with this application which is incorporated herein by reference.
The present invention relates in general to illuminating graphic symbols on display panels, and more specifically to the use of fluorescent pigments to modify color spectrum of illuminated graphics on a portion of a display.
Backlit control panels are used in various devices such as automobile instrument panels, instrument clusters, audio systems, and door switch pads. Backlit graphics on these control panels are typically adjacent or contained on control buttons or dials, such that the graphic indicates the function performed by the buttons or dials. A typical control panel includes a front cover or bezel having translucent graphics contained within an opaque background, a light source behind the bezel, and a light distribution network behind the bezel.
In addition to backlighting, certain displays, such as instrumentation gauges in automobiles, use front projection lighting of symbols or indicators. Light from a light source is usually redirected from behind or from the side of the display toward the front surface of the illuminated graphic.
In both backlit or projection-lit displays, a colored graphic display is achieved by coloring the graphic itself or illuminating it with colored light. For example, many automotive audio systems use white transparent graphics on a black background for most of the bezel graphics together with some number of specially colored transparent graphics to highlight corresponding control functions. The white color facilitates daytime viewing. Under non-daylight conditions, the white graphics are backlit with colored light for a more pleasing appearance.
One possible light source for a backlit control panel is an electroluminescent (EL) lamp panel. An EL panel typically comprises a micro-encapsulated phosphor contained between two conductive plates. One plate is transparent to pass light generated when the plates are charged. EL has advantages of small size (it is very thin) and an ability to selectively backlit desired areas of control panels without complicated light baffles or light pipes. One limitation of EL is that its luminance level or brightness tends to be lower than that achievable with other sources unless high voltage levels or frequencies are applied. Furthermore, the brightness achievable at a particular voltage and frequency depends on the color of light produced. Phosphors formulated for producing blue light have achieved higher brightness than for other colors (especially red) or blends of colors. Thus, brightness from an EL lamp panel has been approximately directly proportional to the frequency of light produced.
Although brightness can be generally increased by applying a higher voltage or a higher frequency to the EL lamp panel, voltage is limited in automotive systems without expensive converters. Higher voltages also create more stress in the phosphor layer, reducing the lifetime and durability of the EL lamp. In addition, higher frequencies are undesirable because of increased electromagnetic radiation. These factors have caused problems in creating multi-colored EL displays.
Another choice for a light source to be used in backlighting is the incandescent bulb. In order to distribute light to the back side of the desired graphics, a light pipe typically having the form of a flat sheet with predetermined reflective surfaces is employed between the incandescent lamp and the bezel.
Since incandescent bulbs produce substantially white light, filtering is used to provide colored light. Control panels with multi-colored incandescent backlighting have been complex due to the need for either colored bulbs or numerous filters. As previously mentioned, a typical control panel utilizes white translucent graphics for good daytime visibility together with some number of specially colored translucent graphics to highlight certain functions. For example, red indicates hot and blue indicates cold for adjustment dials in a climate control panel for an automobile. In order to reduce complexity of a control panel, a colored filter for producing the colored light of most of the graphics (e.g., a green light for backlighting the white translucent graphics) is combined in a single structure with the light pipe for distributing all the light. The same color filtering light pipe directs light (e.g., now colored green) toward the specially colored (e.g., red and blue) graphics for nighttime illumination. However, depending on the colors involved, the specially colored graphics may receive inadequate illumination brightness since there may be little light brightness left at the special color (e.g., red) after the initial filtering (e.g., green).